How can I turn off gamma correction in 2.79b?
It is not gamma correction that you are observing here. It is the rendering process itself. 

This is fundamental part of the process and you cannot do anything about it. Color inputs describe various properties of surfaces and the rendering process attempts to calculate the colors that would be captured by a camera. An object of some specific color will have different colors on its surface depending on how it is lit, the glossiness of the surface, the angle it is viewed from and many many other factors. If you took a sample of some paint for example and you knew the matching reflectance values for the primary colors of your chosen color space and took a photograph of the sample and then viewed that photograph on your screen the color values would not match, because of shadows and highlights on the surface as well as lighting conditions. You cannot and should not try to avoid that. What you can do is try and match the color differences in relation to each other as much as it is possible (which it also actually isn't in practise, but that can be a good goal that makes sense).